1. Field of the Invention
This invention relates to a process chamber capable of performing a variety of thermally driven processes and plasma enhanced processes, such as those involved in semiconductor wafer, flat panel display and hard disk manufacturing. More particularly, this invention relates to maintaining one or more rotating substrates within a controlled environment while injecting particular gases to produce the desired process results.
2. Brief Description of the Prior Art
There are a large number of processes that are performed at elevated temperatures inside of enclosed chambers (usually quartz furnace tubes) wherein the pressure, temperature and composition of gases are precisely controlled to produce the desired process results. Many of the processes performed in this fashion are similar for both semiconductor wafer and flat panel display manufacture and the fabrication of other devices on a wide variety of other substrates. For convenience, hereinafter the term wafer will be used with the understanding that the following would apply to the manufacture of flat panel displays and other types of substrates or devices wherein thermally driven (such as alloying, diffusion, annealing and glass reflow), CVD (Chemical Vapor Deposition) and/or PECVD (Plasma Enhanced Chemical Vapor Deposition) processes are employed.
For instance, silicon nitride is typically deposited on a wafers by CVD processing in a hot wall CVD reactor, as depicted in FIG. 1 and described by S. Wolf and R. N. Tauber, xe2x80x9cSilicon Processing for the VLSI Era, Volume 1xe2x80x94Process Technologyxe2x80x9d, Lattice Press, 1986, pp. 191-194. As shown in FIG. 1, a quartz-walled reactor 1 includes a quartz chamber 10 housing a boat 12 containing a plurality of wafers 14 which are heated by a furnace 16 having multiple zones of resistance heated elements 16a-16c. Boat 12 typically contains between 80-150 of wafers 14. A silicon nitride film is formed via chemical reaction on each of wafers 14 by injecting from tanks 17 gaseous silane or dichlorosilane and either nitrogen or ammonia into chamber 10 via an injection port 18. Other films such as polysilicon, epitaxial silicon, metals, silicides, and glasses may be formed on wafers 14 in a similar fashion by injecting the appropriate reactant gases into the chamber. These reactant gases are removed from chamber 10 by an exhaust port 19. Still other layers may be grown on the heated wafers such as silicon dioxide by injecting oxygen and/or water vapor into the chamber where the oxygen reacts directly with the surface of the silicon wafer to produce the SiO2.
Such a hot wall CVD chamber 10, as illustrated in FIG. 1, is problematic for numerous reasons. First, during CVD processing of wafers 14, the interior surface of the quartz walls of chamber 10 becomes sufficiently hot so as to enable the deposition of the reactant species thereon. As the deposited layer of reactant vapor becomes thicker, pieces thereof may flake off the quartz walls of chamber 10 and contaminate wafers 14. Further, the formation of such a layer may deplete the reactant species such that little or no vapor deposition occurs on the wafers 14. Consequently, the interior surface of the quartz walls of chamber 10 must be periodically cleaned, for example by using a wet etchant, which involves the complete removal of the quartz chamber from the furnace. Although some in the industry suggest alleviating this problem by positioning liners within chamber 10, the liners are equally prone to flaking and must either be cleaned or replaced on a regular basis.
Note that as shown in FIG. 1, the reactant gases are typically injected into a first end of chamber 10 (via injection port 18) and removed from another end of chamber 10 (via exhaust port 19). As a result, the reactant gases are depleted as they travel through chamber 10 such that the deposition rate of the reactant vapor on those of wafers 14 proximate to injection port 18 is higher than those of wafers 14 proximate to exhaust port 19. This phenomenon is known in the industry as the xe2x80x9cgas depletion effectxe2x80x9d and may result in unacceptable variations between films formed on the plurality of wafers 14.
Another problem associated with hot wall CVD reactors such as reactor 1 is the significant time required both before and after processing of wafers 14. Pre-processing time includes the time required to load the plurality of wafers 14 into boat 12, insert boat 12 into chamber 10, and slowly ramp up the temperature within chamber 10 from a loading/unloading temperature to a constant and uniform process temperature. Post-processing time includes the time required to slowly ramp the temperature down from the process temperature to the loading/unloading temperature, to remove boat 12 from chamber 10, and to further cool boat 12 and wafers 14 therein to allow wafers 14 to be loaded into plastic wafer cassettes (not shown). The slow insertion rate of boat 12 into chamber 10 and removal rate of boat 12 from chamber 10, as well as the slow temperature ramp up before processing and slow temperature ramp down after processing, are necessary to ensure minimal temperature gradients across the surface of wafers 14, thereby avoiding wafer warping and/or crystal slippage of wafers 14. While actual processing of wafers 14 may require only one-half hour, the time required for the pre- and post-processing procedures just discussed is typically 1 to 2xc2xd hours. Accordingly, this pre- and post-processing time significantly limits the throughput of chamber 10. Further, the slow temperature ramp up, the slow process of obtaining a uniform and constant temperature across the surface of wafers 14, and the slow temperature ramp down result in chamber 10 contributing a relatively high thermal budget (the time that the wafers are above room temperature) to wafers 14. As the size of semiconductor devices continues to decrease, it becomes increasingly important to minimize the thermal budget.
Although employing boat 12 capable of holding a large number of wafers helps to maximize throughput of chamber 10, the simultaneous processing of so many of wafers 14 increases risk of wafer loss should something go wrong during processing. For instance, if a gas flow controller malfunctions or the vacuum pump ceases operating properly, all of wafers 14 in boat 12 may be destroyed.
Further note that where compatible process steps are to be performed on a wafer sequentially, it is often desirable to implement automated sequential processing in order to increase throughput. However, the relatively large size of chamber 10 and the large number of wafers 14 contained in boat 12, coupled with the necessarily slow loading and unloading rate of boat 12 at atmospheric pressure, makes automated sequential processing in chamber 10 very cumbersome, if not impractical.
An improved reaction chamber is disclosed which overcomes problems in the prior art described above. In accordance with the present invention, the reactor includes a vacuum chamber having two distinct sections. The lower section is used for the loading and unloading of the wafers and the upper section is where the temperature, pressure and the flow of gases can be precisely controlled to produce desired uniform and consistent process results. The reactor is connected to a central transfer vacuum chamber via a slit valve such that the wafers can be loaded into the wafer boat while under vacuum via the robotic arm of the transfer chamber. The wafer boat is supported by a shaft which, being movable in a vertical direction, allows wafers to be loaded/unloaded into the various slots of the wafer boat. Once all the wafers are loaded into the boat, the slit valve is closed and the boat containing the wafers is elevated into the upper section of the chamber. The wafer boat and wafers therein are heated by multiple zone radiant heaters e.g. tungsten halogen lamps arranged around the periphery of the upper section of the chamber and by additional heaters positioned about the top and bottom surfaces of the chamber. In this manner, a desired thermal profile may be obtained. Thermocouples inserted into a high temperature material such as graphite or silicon carbide are positioned at strategic positions within the chamber and provide temperature feedback to the controller (or computer) which, in turn, controls the various heater zones. In other embodiments, pyrometry or other means may provide this temperature feedback.
Once inserted into the upper section of the chamber, the boat and wafers therein are rotated (typically 3 to 30 rpm). After the desired wafer temperature has stabilized, the process gases are injected into and exhausted from the chamber in a cross flow fashion parallel or nearly parallel to the wafers so as to result in a uniform process results. The flow rate of the process gases may be adjusted as function of vertical position relative to the wafer boat via independently controlled gas injection and exhaust ports. In this manner, the ability to optimize the process gas flow pattern within the chamber is realized, irrespective of the particular process gas or gases employed and irrespective of the process pressure.
A plurality of electrically conductive shield plates surround the interior walls of the chamber. Where it is desired to perform PECVD processes and in situ plasma cleaning, RF (radio frequency) power may be applied to a first number of the plates while a second number of the plates are held at ground potential. Further, the shield plates prevent the deposition of the reactant species onto the quartz windows of the chamber and also serve to diffuse heat emitted from the heating elements.
The reaction chamber described below could easily be a stand alone product (without the bottom portion), where the entire boat load of wafers or flat panel displays is inserted into and removed from the reactor instead of single wafer load/unload as described below. Thus, the reaction chamber could be a xe2x80x9cmanual boat load/unloadxe2x80x9d reactor (or simple mechanical load/unload) instead of the robotic load/unload system described below. Moreover, the entire machine could be flipped over such that wafer load/unload is accomplished in the upper portion and process in the lower portion. The preferred embodiment is described for bottom load/unload to keep the wafer transport plane (robotic arm height) closer to the floor which is more consistent with other semiconductor process equipment such as single wafer cluster tools.
The gas injection and exhaust systems of this invention are important features. For instance, in many types of CVD furnace reactors, a quartz tube with small holes up and down its length is inserted into the main quartz tube adjacent to the wafers and the process gases are introduced via the holes in this smaller tube in an effort to overcome gas depletion effects to achieve better uniformity and higher deposition rates throughout the boat load of wafers. However, since the smaller tube is in an isothermal environment with the wafers and is at the same deposition temperature, the CVD material being deposited on the wafers is also being deposited on the edges of these holes and in a relatively short period of time, this deposit will start clogging the holes and can flake off and fall onto the wafers. Further, as one tries tc increase the flow of the reactant gases in these smaller tubes in order to achieve even higher deposition rates on the wafers, the partial pressure within the tube increases which in turn accelerates the deposition on the tube""s walls and at the holes. Because the present invention utilizes a cooled gas injection scheme and because the gas injection rate can be tuned up and down the wafer load, the deposition rates achievable in the reactor of this invention can be from 5 to 30 times greater than that for a typical furnace type reactor, while still maintaining desired uniformity from wafer to wafer and across each wafer. The tuning of the exhaust further enhances the ability to achieve maximum deposition rates while maintaining deposition uniformity on the wafers. Further, the present invention provides for the heating of the exhaust plate and manifold which is desirable for many CVD processes to prevent condensation which can also lead to the generation of particles that can migrate to the wafers. Finally, since the injection and exhaust plates are merely bolted onto the chamber, they can be easily removed and replaced if necessary.
The plasma capability of the present invention is of significance. In addition, the way the RF energy is coupled to opposing sets of side shield plates in conjunction with the presence of the upper and lower shield plates makes for a nearly symmetrical geometry which is important to maximize the plasma""s energy uniformity. This design also allows for switching the RF from side to side which aids in the uniformity of material removal during in situ plasma cleaning. The RF input assembly is designed with dark space shielding to eliminate the creation of a plasma behind the shield plate clips which otherwise would cause hot spots in that area that would detract from the energy and uniformity of the plasma in the cleaning region of the chamber.
The cold wall construction of the present invention: a) allows for use of multiple zone radiant heating with simple and inexpensive quartz windows (the total cost of the quartz parts of the present invention is one-tenth or less the cost of replacing the quartzware of a conventional vertical furnace); b) allows for cluster tool configuration for automatic load/unload under vacuum and for sequential processing from one reactor (process A) to another (process B) and so on; c) reduces deposition on the cooled walls; d) eases attachment/removal of sub-assemblies such as the side heaters, gas injection, exhaust manifold and RF/TC seal plates; and e) permits accelerated temperature ramp down when the pressure within the chamber is increased with inert gas flow to permit greater heat conduction through the gas to the cold walls.
The multi-zone radiant heating of the present invention provides: a) much lower thermal inertia compared to a resistively heated furnace, resulting in faster response time; b);as many as ten to twelve independent heat zones (compared to 5 to 7 for a typical vertical furnace); and c) the ability to tailor the thermal pattern within the chamber for faster temperature ramping without wafer warpage or crystal slippage.
The wafer rotation feature of the present invention promotes better uniformity across a wafer and from wafer-to-wafer, while permitting faster deposition rates (up to 5 to 30 times faster deposition rates than a conventional furnace).
The low thermal mass-shielded plates of the present invention have the following advantages: a) the bottom, top and side shield plates entirely surround the wafers and serve to diffuse the heat energy from the lamps for greater uniformity; b) the low thermal mass permits rapid cycling of temperature; c) the existence of these plates allows for inert gas to be injected between the plate and the quartz window to prevent the reactive process gases from penetrating and causing deposition on the window; and d) the shield plates are made of materials (graphite, silicon carbide, ceramic, etc.) that result in a very high level of adhesion of typical CVD films even during temperature cycling compared to a quartz furnace type reactor where particles of the deposited material on the quartz can easily flake off and onto wafers.
The bottom shield plate of the present invention: a) moves up and down with the wafer boat; b) can be left floating or grounded or connected to the RF energy source during plasma processing; c) provides a degree of thermal isolation between the upper process chamber and the lower load/unload chamber; d) with inert gas flow into the lower chamber, the plate serves to effectively retard the process gases from entering the lower chamber; e) the existence of the plate greatly enhances the ability to achieve a uniform gas flow pattern in the process chamber by serving as a bottom plate to the process chamber such that the top plate and the bottom shield are approximately equidistant with respect to the boat of wafers, and f) diffuses the heat from the bottom heaters.